High frequency oscillator having grid direct current stabilizing means



circuit arrangement.

United States Patent HIGH FREQUENCY OSCILLATOR HAVING GRID DIRECT CURRENT STABILIZING MEANS Lourens Blok and Gooitzen Zwanenburg, Eindhoven,

Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Apr. 18, 1958, Ser. No. 729,470 Claims priority, application Netherlands May 2, 1957 6 Claims. (Cl. 331-183) The present invention relates to a high-frequency oven More particularly, the invention relates to a high-frequency oven circuit arrangement comprising a feedback connected grid-controlled electron tube oscillator, in which the control grid D.C. circuit of the electron tube oscillator includes a pentode connected as a variable resistor between the control grid and the. cathode of the back-coupled electron tube for the purpose of stabilizing the grid direct-current upon varying load.

In such high-frequency ovens, the direct-current stabilization of the control grid has the effect that an advantageous control of the feedback connected electron tube is obtained under any operating conditions, that is to say between no load and full load.

According to the invention, one resistor is arranged in parallel with the pentode which is connected as a variable resistor. Between the cathode of the pentode and the control grid of the feedback connected electron tube oscillator there is included a resistor, of which a point of junction remote from the cathode of the pentode is connected via a constant voltage source of positive polarity to the control grid of the pentode. The invention ensures that the pentode included in the control-grid D.C. circuit of the oscillator tube with effective stabilization of the grid direct-current, need be proportioned only for a fraction of the maximum grid directcurrent of the high-frequency oven, which permits of using pentodes of considerably lower output for the stabilization of the grid direct-current.

In order that the invention'may be readily carried into effect, it will now be described in detail, by way of example, with reference to the accompanying drawing, in which:

Fig. 1 shows a high-frequency oven according to the invention;

Fig. 2 shows several characteristic curves to explain the high-frequency oven of Fig. 1.

The circuit diagram shown in Fig. 1 relates to a highfrequency oven of high power, for example 12 kilowatts, in which the high-frequency energy is derived from a gridcontrolled electron tube oscillator having a triode 1. Theanode circuit of the oscillator, which is a Colpitts circuit, includes an oscillatory circuit 2, determining the frequency of the oscillator, and a circuit coil 3 which is bridged by two capacitors 4 and 4 connected in series and the junction of which is connected to the cathode of triode 1. One end of the anode circuit is connected via a capacitor 9 to the anode of tube 1, the positive feedback voltage derived from the extremity of the oscillatory circuit remote from the anode is supplied through a grid capacitor 5 to the control grid of tube 1.

The anode of tube 1 is fed through a high-frequency choke coil 6 by means of a direct current supply source 7, which is shunted by a high-frequency decoupling capacitor 8.

The circuit current which occurs in the oscillatory circuit 2 during oscillation of the circuit is used for heating a work-piece 11, which is included in the load circuit constituted, for example, by a heating coil 10. The circuit 2 is coupled by inductive means to a coupling coil 12, to the output terminals of which the heating coil 10 is connected.

For stabilizing the grid direct-current of the high-frequency oven upon variable load, the control-grid D.C. circuit of the high-frequency oscillator includes a pentode 15 which is connected as a variable resistor. The pentode is bridged by a resistor 14. The cathode of the pentode is connected via a variable resistor 16 and a high-frequency choke coil 17 to the control grid of the triode 1. One point of junction of the variable resistor 16 is coupled via a constant biassing source of positive polarity and a series-resistor 28 to the control grid of pentode 15. The constant biassing source of positive polarity is constituted by a gas-filled tube 18, which is connected via a resistor 19 to the cathode of the oscillator triode. In order to prevent the anode dissipation of pentode 15 from exceeding a determined permissible limiting value, the anode circuit of the pentode includes a series-resistor 20.

The grid voltage V of the pentode is given by the constant operating voltage V of the gas filled tube, for example volts, less the voltage drop brought about by the grid direct-current I of triode 1 across resistor 16, which has a value R. In the form of a formula, the grid voltage V is:

If, as a result of variation in the load the high-frequency oven, the grid direct-current of the triode 1 varies, the grid voltage V of the pentode varies in the opposite sense with respect to the variation in the voltage drop across the resistor. The resistor constituted by the pentode 15 varies in the same sense as the variation in the grid direct-current, with the result that the variation in the resistor constituted by the pentode counteracts the variation in the grid direct-current of the triode 1.

The variable grid resistor 13 thus has a maximum value with maximum grid direct-current of the triode 1, that is to say in the unloaded condition of the high-frequency oven, thus value decreasing upon increasing load on the high-frequency oven. In the embodiment shown, the pentode 15 is adjusted by means of resistor 16 so that the pentode 15 is substantially cut off in the unloaded condition, that is to say with maximum grid direct-current. the total grid current then being substantially absorbed by the resistor 14 connected in parallel with the pentode.

Fig. 2 shows the variation of the currents I which occur in the variable grid resistor 13 as a function of the load W on the high-frequency oven. The curves a and b representing the direct currents absorbed by the pentode 15 and the resistor 14, connected in parallel with the pentode, and the curce c representing the total grid current of the triode 1. The current characteristics shown relate to the 12 kw. high-frequency oven of Fig. 1, which has been extensively tested in practice.

As previously mentioned, the pentode 15 is substantially cut oft in the unloaded conditions, whilst the resistor 14 connected in parallel with the pentode 15 absorbs substantially the whole grid current of triode 1. In this condition, the points 21 and 22 on the curves [1 and b represent the currents absorbed by the pentode 15 and the resistor 14, which currents are 10 milliamps, and 3l0 milliamps, respectively.

When the load on the high-frequency oven increases, the current through the pentode 15 increases and the current through resistor l4rdecreases. With full load The greatly varying anode voltage of the pentode has substantially no'influence upon the control of pentode 15.

The variation of the total grid current of the highfrequency oven, which is equal to the sum of the currents traversing the pentode 15 and the resistor 14, is represented by the curce c in Fig. 2. Between no load and full load on the high-frequency oven, the total grid current thus varies only from 320 to 280 milliamps.

The arrangement of the invention permits of obtaining effective stabilization of the grid direct-current of the high-frequency oven under any operating conditions; that is, between no load and full load. This object is attained with the use of the tube 15 which absorbs only a fraction of the total grid current, so that a considerably smaller type of tube is sufficient. In fact, in the embodiment shown, the pentode 15 need absorb only 160 milliamps. of the-total grid current, which is at the most 320 milliamps. A considerable saving is thus obtained especially for high-frequency ovens of high output, for example upwards of 3 kilowatts.

In this connection, it is to be noted that, instead of utilizing a single pentode 15 in the grid D.C. circuit, it is also possible to utilize a plurality of parallel-connected pentodes of smaller type.

In the embodiment shown, the screen-grid voltage of pentode 15 is derived from a separate source of supply voltage, the positive terminal of which is connected to the screen grid of pentode 15 and the negative terminal of which is connected to the cathode. In order to avoid undue increase in screen-grid current when the anode voltage of pentode 15 drops out, for example upon switching off the high-frequency oven, a resistor 26 is included between the negative terminal of the supply voltage source and the cathode of pentode 15 and the negative terminal is connected via a resistor 27 to the control grid. In fact the resistor 27, together with the resistors 19, 20, 14, constitutes a voltage divider connected in parallel with resistor 26 of the cathode circuit and of which the junction of resistors 19 and 27 is connected to the control grid of pentode 15. In this condition the gas-tube 18 is extinguished. When in this circuit the screen-grid current increases, the control-grid voltage decreases, with the result that an undue increase in screen-grid current is counteracted.

In conclusion, several data of the equipment shown in Fig. 1 follow hereinafter:

Resistor 14 ohms 2000 Resistor 16 do 680 Resistor 20 do 430 Resistor 19 do 91K Resistor 26 -do 1K Resistor 27 do 100K Type of gas tube 18 85A2 Operating voltage of gas tube 85 volts What is claimed is:

l. A high frequency oven circuit arrangement, comprising a feedback connected grid-controlled electron tube oscillator including an electron tube having cathode, anode and control grid electrodes, a variable load coupled to the anode of said electron tube, means responsive to the flow of grid current for producing a biasing potential at said control grid electrode, said grid current responsive means comprising means for stabilizing the grid direct current of said oscillator upon variation of said load, said stabilizing means comprising a pentode having cathode, anode and control grid electrodes, means connecting said pentode between the control grid electrode and the cathode of said electron tube and means for varying the effective resistance of said pentode in a sense diminishing said resistance upon reduction of said lbiasing potential comprising a first resistor connected in parallel with said pentode between the cathode and anode of the said pentode, a second resistor connected between the cathode of said pentode and the control grid electrode of said electron tube, a source of constant voltage of positive polarity and means connecting a terminal of said second resistor between the said second resistor and said electron tube to the control grid electrode of said pentode through said source of constant voltage.

2. A high frequency oven circuit arrangement, comprising a feedback connected grid-controlled electron tube oscillator including an electron tube having cathode, anode and control grid electrodes, a variable load coupled to the anode of said electron tube, means responsive to the flow of grid current for producing a biasing potential at said control grid electrode, said grid current responsive means comprising means for stabilizing the grid direct current of said oscillator upon variation of said load, said stabilizing means comprising a pentode having cathode, anode and control grid electrodes, means .con necting said pentode between the control grid electrode and the cathode of said electron tube and means for varying the effective resistance of said pentode in a sense diminishing said resistance upon reduction of said biasing potential comprising a first resistor connected in parallel with said pentode between the cathode and anode of the said pentode, a second resistor connected between the cathode of said pentode and the control grid electrode of said electron tube, said second resistor being proportioned to substantially cut off said pentode when said grid current has a maximum magnitude, a source of constant voltage of positive polarity and means connecting a terminal of said second resistor between the said second resistor and said electron tube to the control grid electrode of said pentode through said source of constant voltage.

3. A high frequency oven circuit arrangement, comprising a feedback connected grid-controlled electron tube oscillator including an electron tube having cathode, anode and control grid electrodes, a variable load coupled to the anode of said electron tube, means responsive to the flow of grid current for producing a biasing potential at said control grid electrode, said grid current responsive means comprising means for stabilizing the grid direct current of said oscillator upon variation of said load, said stabilizing means comprising a pentode having cathode, anode and control grid electrodes, means conmeeting said pentode between the control grid electrode and the cathode of said electron tube and means for varying the effective resistance of said pentode in a sense diminishing said resistance upon reduction of said biasing potential comprising a first resistor connected in parallel with said pentode between the cathode and anode of the said pentode, a second resistor connected between the cathode of said pentode and the control grid electrode of said electron tube, a source of constant voltage of positive polarity comprising a gas tube, means connecting a terminal of said second resistor between the said second resistor and said electron tube to the control grid electrode of said pentode through said source of constant voltage and means connecting said source of constant voltage to the anode of said pentode, said last-mentioned means including a resistor.

4. A high frequency oven circuit arrangement, cornprising a feedback connected grid-controlled electron tube oscillator including an electron tube having cathode, anode and control grid electrodes, a variable load coupled to the anode of said electron tube, means responsive to the flow of grid current for producing a biasing potential at said control grid electrode, said grid current responsive means comprising means for stabilizing the grid direct current of said oscillator upon variation of said load, said stabilizing means comprising a pentode having cathode, anode and control grid electrodes, means connecting said pentode between the control grid electrode and the cathode of said electron tube and means for varying the effective resistance of said pentode in a sense diminishing said resistance upon reduction of said biasing potential comprising a first resistor connected in parallel with said pentode between the cathode and anode of the said pentode, a second resistor connected between the cathode of said pentode and the control grid electrode of said electron tube, a source of constant voltage of positive polarity, means connecting a terminal of said second resistor between the said second resistor and said electron tube to the control grid electrode of said pentode through said source of constant voltage, and a third resistor connected in series between the anode of said pentode and the cathode of said electron tube.

5. A high frequency oven circuit arrangement, comprising a feedback connected grid-controlled electron tube oscillator including an electron tube having cathode, anode and control grid electrodes, a variable load coupled to the anode of said electron tube, means responsive to the flow of grid current for producing a biasing potential at said control grid electrode, said grid current responsive means comprising means for stabilizing the grid direct current of said oscillator upon variation of said load, said stabilizing means comprising a pentode having cathode, anode and control grid electrodes, means connecting said pentode between the control grid electrode and the cathode of said electron tube and means for varying the effective resistance of said pentode in a sense diminishing said resistance upon reduction of said biasing potential comprising a first resistor connected in parallel with said pentode between the cathode and anode of the said pentode, a second resistor connected between the cathode of said pentode and the control grid electrode of said electron tube, said second resistor being proportioned to substantially cut oil? said pentode when said grid current has a maximum magnitude, a source of constant voltage of positive polarity comprising a gas tube, means connecting a terminal of said second resistor between the said second resistor and said electron tube to the control grid electrode of said pentode through said source of constant voltage, means connecting said source of constant voltage to the anode of said pentode, said lastmentioned means including a resistor, and a third resistor connected in series between the anode of said pentode and the cathode of said electron tube.

I 6 6. A high frequency oven circuit arrangement, comprising a feedback connected grid-controlled electron tube oscillator including an electron tube having cathode, anode and control grid electrodes, a variable load coupled to the anode of said electron tube, means responsive to the flow of grid current for producing a biasing potential I at said control grid electrode, said grid current responsive means comprising means for stabilizing the grid direct current of said oscillator upon variation of said load, said stabilizing means comprising a pentode having cathode, anode, control grid and screen grid electrodes, means connecting said pentode between the control grid electrode and the cathode of said electron tube and means for varying the effective resistance of said pentode in a sense diminishing said resistance upon reduction of said biasing potential comprising a first resistor connected in parallel with said pentode between the cathode and anode of the said pentode, a second resistor connected between the cathode of said pentode and the control grid electrode of said electron tube, a source of constant voltage of positive polarity, means connecting a terminal of said second resistor between the said second resistor and said electron tube to the control grid electrode of said pentode through said source of constant voltage, a source of screen grid supply voltage having a positive terminal connected to the screen grid electrode of said pentode and a negative terminal, a third resistor, means connecting said negative terminal to the cathode of said pentode through said third resistor and a voltage divider connected in parallel with said third resistor, said voltage divider including said first resistor and having a tapping point connected to the control grid electrode of said pentode.

References Cited in the file of this patent UNITED STATES PATENTS 2,373,437 Vanderlyn et al. Apr. 10, 1945 2,501,834- Wheeler Mar. 28, 1950 2,540,638 Williams Feb. 6, 1951 2,584,850 De Mers Feb. 5, 1952 2,589,477 Corporon Mar. 18, 1952 

